I am looking for some advice on how to organise my C++ code.
I have an int array, side, that I would like to be static in the sense that its value is kept constant between calls. This is because my function foo(), will modify the array side recursively and so I don't want copies to be made of side. Furthermore, the size of side can only be determined at compile time from the size of a vector that is passed into the function bar().
I have thought of the following structure to layout such a problem.
I keep a global int pointer, side, which I can then use to point to the address of my int array and then use the pointer *side within foo to do my modifications.
Please can you give me advise on the layout and organisation of this code? I am quite new to C++ so would appreciate any advice on the below structure.
#include <iostream>
#include <vector>
using namespace std;
int *side;
class A {
public:
int foo(bool);
int bar(vector<int>);
void set_n(int n){ class_n = n;};
private:
int class_n;
};
int A::foo(bool fl)
{
int n = class_n;
for(int i = 0; i < n; i++) {
// modify side[] and then recursively call foo
}
return 0;
}
int A::bar(vector<int> t)
{
int size = t.size();
set_n(size);
int a = foo(true);
int *side_local = new int[size];
for(int i = 0; i < size; i++) {
side_local[i] = 0;
}
side = side_local;
return 0;
}
int main()
{
A a;
vector<int> t = {1, 2, 3};
a.bar(t);
return 0;
}
A recursive call can pass a pointer to itself:
void foo(int *pList)
{
foo(pList); // recursive
}
the same list is then being worked on.
That being said, since foo is inside a class you wouldn't need a global either, but a member variable.
class A
{
int *pMemberList;
...
void foo();
}
now foo can see pMemberList all the time.
BUT ... passing it is probably a better option as in the future your class might house 2 lists that you want to do foo on.
Related
I am trying to swap the content in the arrays by swapping the pointers pointing to the two arrays.
My method is the same as what Daniel answered in this question: Swap arrays by using pointers in C++. But the difference is that my array will be a member in a class.
My code can be compiled successfully, but the output results are quite weird.
This is my header file:
#include <stdio.h>
#include <iostream>
class Map
{
public:
Map(int times); // Create an empty map (i.e., one with no key/value pairs)
int size(); // Return the number of key/value pairs in the map.
void dump();
void swap(Map &other);
int *retrieve();
void setptr(int *newptr);
private:
int *ptr;
int array_1[5];
};
Here is my implementation:
#include "Map.h"
#include <iostream>
using namespace std;
Map::Map(int times) {
for (int i = 0; i < 5; i++) {
array_1[i]=i*times;
}
ptr=array_1;
}
void Map::dump() {
ptr=array_1;
for (int i = 0; i < 5; i++) {
cout << *ptr << endl;
ptr++;
}
for (int i = 0; i < 5; i++) {
ptr--;
}
}
void Map::swap(Map &other) {
int *temp;
temp = this->ptr;
this->ptr = other.retrieve();
other.setptr(temp);
}
int *Map::retrieve() {
return ptr;
}
void Map::setptr(int *newptr) {
ptr=newptr;
}
Can anyone tell me what is wrong and how to implement it smartly?
The following code runs fine:
#include <stdio.h>
#include <iostream>
#include <conio.h>
using namespace std;
class Map
{
public:
Map(int times); // Create an empty map (i.e., one with no key/value pairs)
int size(); // Return the number of key/value pairs in the map.
void dump();
void swap(int &other);
int *retrieve();
void setptr(int *newptr);
private:
int *ptr;
int array_1[5];
};
Map::Map(int times){
for (int i=0;i<5;i++){
array_1[i]=i*times;
}
ptr=array_1;
}
void Map::dump(){
for (int i=0;i<5;i++)
{
cout<<ptr[i]<<endl;
}
}
void Map::swap(int &other){
int *temp;
temp=this->ptr;
this->ptr=&other;
other = *temp;
}
int *Map::retrieve(){
return ptr;
}
void Map::setptr(int *newptr){
ptr=newptr;
}
int main()
{
Map m(2);
Map n(3);
m.dump();
m.swap(*n.retrieve());
m.dump();
getchar();
}
1) Added a main function
2) Changed Swap function
But the problem that christopher pointed out will still persist i.e the pointer will point to an array in another object.
Edit: You probably need something like this:
void Map::swap(Map &other){
Map *temp;
temp=this;
*this = other;
other = *temp;
}
Map *Map::retrieve(){
return this;
}
Note: it is probably not elegant.
The problem with your design is that the pointer refers to an array in the same object.
Suppose you have to objects a and b. If you swap their pointers, a.ptr will point to b.array_1 which contains the data. reciprocally b.ptr will point to a.array1.
Unfortunately if one of the object -- say b -- gets destroyed (because it was a local object that goes out of scope, or for whatever reason) the pointer of the remaining object would point to an array which doesn't exist anymore. This is UB.
To solve your issue, you'd neet to allocate an array dynamically in the constructor. Get rid of array_1 completely:
Map::Map(int times){
ptr=new int[5]; // or better define a constant to avoid hard coded sizes
for (int i=0;i<5;i++){
ptr[i]=i*times;
}
}
Note that if you use pointers, you need to ensure the invarients on it. This means that you should define also the copy constructor and the assignment operator (to avoid the ptr to be blindly copied), as well as a destructor (to delete the dynamically allocated array).
P.S.: I suppose that you are learning C++ and are not yet familiar with vectors. These would avoid all the hassles here
Edit: if you experience your problem before any object is destroyed, it's because of a bad implementation of dump(): you increment the pointer there in, so that it will no longer point to the start of the array.
void Map::dump(){
for (int i=0;i<5;i++){
cout<<ptr[i]<<endl; // don't change ptr value !!
}
}
One simple trick to avoid such problems, is to systematically declare the member functions that are not supposed to change the state of the object as const:
class Map {
...
void dump() const;
...
}
Then the compiler issues an error if you try to accidentally change a member.
I need some help on multi-dimensional arrays... I cannot find out how to assign a value to an array in a void task, that was created in main.
i tried to find help all over the place, but the longer i keep reading the less i understand
Please help
void addValue(a,b)
{
//somehow assign value to a[2][6] using pointers and such
void main()
{
int dest[7][7] = { 0 };
int a = 2;
int b = 6;
addValue(a,b);
}
Like this:
void addValue( int (*dest)[7], int a, int b )
{
dest[2][6] = 12;
}
int main()
{
int dest[7][7] = { 0 };
addValue(dest, a, b);
}
Consider using std::array instead of C-style arrays; the latter are an anachronism in C++. You can make the 7 a template parameter in addValue if you want to support other dimensions of array.
I would like to create a struct and use it inside an other struct as an array. My problem is that I don't know how big array I would like to allocate, I will only know once I am in a function. I mean I would like to use [] instead of a pre-determined constant, like 10000.
I think if you look at my code it would be self-explanatory. Can you help me how to make this code work? Moreover it would help me a lot if you could tell me what is the name of the topic I am asking about (is it dynamic arrays?) and that where can I find articles/tutorials about this topic.
Here is the code with my broken way of thinking about arrays in structs.
#include <iostream>
using namespace std;
struct keyframe {
bool a;
int b;
int c;
};
struct keyframe_file {
const int num_views;
const int num_keyframes;
keyframe keyframes[];
};
int main() {
keyframe_file my_file;
my_file.num_views = 1;
my_file.num_keyframes = 6;
my_file.keyframes = new keyframe[my_file.num_keyframes];
my_file.keyframes[0].a = true;
my_file.keyframes[0].b = 5;
my_file.keyframes[0].c = 9;
return 0;
}
Use a std::vector.
struct keyframe_file {
const int num_views;
const int num_keyframes;
std::vector<keyframe> keyframes;
};
int main() {
keyframe_file frame;
frame.keyframes.resize(...);
}
If it suits your purpose, an STL container (std::vector) is easily one of the best options - the less memory management you have to worry about, the better.
In any case, look at the struct definition Nawaz posted above - that's exactly how it should be. Dynamic arrays in C++ are simply pointers. You have, however, allocated the memory properly in your code, but you haven't freed it (so it's leaking). Since you allocated with new [] you will need to
delete [] my_file.keyframes;
in order to free the memory properly.
Resizing is another issue: with a smart implementation, array resizing can be an amortized O(1) operation which is nice. When you resize, it will always take you O(n) since you need to copy all the elements into a new array of different size, but if you do it half as much, it becomes O(1). That is, double the array each time you need to resize. Here is a very quick example
void resize()
{
if(numOfElementsInArray == sizeOfArray)
{
ArrayType * arr = new ArrayType[sizeOfArray*2]; // Allocate a double size array
for(int i=0;i<sizeOfArray;++i)
currentArray[i] = arr[i];
delete [] currentArray; // Free memory in old array
currentArray = arr; // Set the array to our new one
sizeOfArray *= 2; // Double the size
}
}
NOTE: The example above does not take into account space complexity; that said, if you have 5000 elements, and remove all but 5, this method with not shrink it (which is probably what you will want to do for all practical purposes)
Your code appears to be almost correct, except for two things:
keyframes needs to be a keyframe* rather than a keyframe[]
You forgot to delete the memory you allocated
That is incomplete type. In C++, array must be provided with size, and the size must be known at compile time itself.
You're using new, with which you should be using pointer.
struct keyframe_file {
const int num_views;
const int num_keyframes;
keyframe *keyframes;
};
But std::vector<keyframe> is still a better choice, as #DeadMG already suggested.
By the way, the first two members are const in the struct, that means, they cannot be assigned value, as you're doing in your code. They must be initialized with values you want them to hold. That implies, now with vector, you've to include a constructor, to initialize the struct, as the struct is no more a POD.
struct keyframe_file {
const int num_views; //const member
const int num_keyframes; //const member
std::vector<keyframe> keyframes;
keyframe_file(int nviews, int nkeyframes)
: num_views(nviews), num_keyframes(nkeyframes), keyframes(nkeyframes){}
};
keyframe_file my_file(1,6); //done!
The suggested "Vector" is they safest way to do it.
But if it is only about making your code work (without resizing and stuff) the following should be working:
#include <iostream>
using namespace std;
struct keyframe {
bool a;
int b;
int c;
};
struct keyframe_file {
const int num_views;
const int num_keyframes;
keyframe* keyframes;
};
int main()
{
keyframe_file my_file = {1, 6}; // initialization needed bcause of 'const int'
my_file.keyframes = new keyframe[my_file.num_keyframes];
for (int i = 0; i < my_file.num_keyframes; i++)
{
my_file.keyframes[i].a = true;
my_file.keyframes[i].b = 5 + i;
my_file.keyframes[i].c = 9 - i;
}
return 0;
}
somewhere in your code, when you are done using the array you have to call delete [] my_file.keyframes; as already mentioned.
There's a basic rule when using dynamic arrays in c++, especially when using it inside structs or classes, and it's to delete what you no longer need.
If you want to make your struct dynamic, it's easy, just replace the [] with * and the array will become dynamic, but it's not over yet, there is a lot of work.
You have to construct the array and destory it, and destoroying it is possible and useful noly with destructors, like this:
struct keyframe_file
{
const int num_views;
const int num_keyframes;
keyframe* keyframes;
~keyframe_file() // this is the destructor
{
delete[] keyframes;
}
};
Yet even that code isn't going to work at all, since you are assigning values to constants in variable my_file after creating it, it's illegal in c++, you should then use classes instead.
Using classes with dynamic arrays is very easy and interesting and makes your code very good, you don't have to know too much to do that, just learn what is a constructor, an initializer, destructor, private and public and go on with the following code:
#include <iostream>
using namespace std;
struct keyframe
{
bool a;
int b,c;
};
class keyframe_file
{
public:
keyframe_file(int NV, int NKF):num_keyframes(NKF),num_views(NV)
{
keyframes = new keyframe[num_keyframes];
}
~keyframe_file()
{
delete[] keyframes;
}
private:
const int num_views;
const int num_keyframes;
keyframe* keyframes;
};
int main()
{
keyframe_file my_file(1,6);
return 0;
}
This code works very well, it allows you to assign value to the constants num_views and num_keyframes for one time when creating the object (variable) my_file.
Remember, you are a C++ programmer, be proud of that, and use classes instead of structs and dynamic arrays instead of static ones.
Hope that's useful.
Use pointers and apply to your structure!
int *p;
p = new int;
#include <iostream>
using namespace std;
struct keyframe {
bool a;
int b;
int c;
};
struct keyframe_file {
const int num_views;
const int num_keyframes;
keyframe *keyframes;
};
int main() {
keyframe_file my_file;
my_file.num_views = 1;
my_file.num_keyframes = 6;
for (int i = 0; i < my_file.num_keyframes; i++){
my_file.keyframes = new keyframe; //<---
}
my_file.keyframes[0].a = true;
my_file.keyframes[0].b = 5;
my_file.keyframes[0].c = 9;
return 0;
}
#include <iostream>
template<class T> T CreateArray(T a, int n)
{
a = new T [n]; // mistake: double* = double**
return a;
}
int main()
{
double* a;
int n = 5;
a = CreateArray(a,n);
return 0;
}
can I allocate memory using a template and new? And what my mistake?
Your code has some wrong things. First, you can do something like what you're trying to do, but you should write something like this:
template<class T> T* CreateArray(int n)
{
T* a = new T [n];
return a;
}
int main()
{
double* a;
int n = 5;
a = CreateArray<double>(n);
return 0;
}
Note that you don't have to pass the a array (it will be copied inside CreateArray, and its changes won't be visible inside main). Note also that you define the template to returning a pointer T*, that is what main() a is expecting.
So others have explained why your code doesn’t work and how it can be improved.
Now I’ll show how you can still get the following code to compile – and to work properly:
double* a = CreateArray(5);
int* b = CreateArray(7);
The problem, as already mentioned, is that C++ does not infer template arguments from return types alone.
You can circumvent this limitation by making the above function return a simple proxy object. The proxy object has a single operation: an (implicit) conversion to T*. This is where the actual allocation happens.
The CreateArray function is therefore very simple (and not a template):
CreateArrayProxy CreateArray(std::size_t num_elements) {
return CreateArrayProxy(num_elements);
}
As for the proxy:
struct CreateArrayProxy {
std::size_t num_elements;
CreateArrayProxy(std::size_t num_elements) : num_elements(num_elements) { }
template <typename T>
operator T*() const {
return new T[num_elements];
}
};
Easy as π.
Now, should you use this code? No, probably not. It offers no real advantage over direct allocation. But it’s a useful idiom to know.
You want to accept a pointer to the type you want to allocate:
template<class T> T* CreateArray(T* a, int n)
{
a = new T [n];
return a;
}
This should do the trick.
I prefer to keep empty pointers value NULL.
#include <iostream>
template<class T> bool CreateArray(T * &a, int n)
{
if ( a != 0 )
return false;
a = new T [n];
return true;
}
int main()
{
double* a = 0;
int n = 5;
CreateArray(a,n);
return 0;
}
vector could be a good solution, too. I think it is better one, because you won't make memory leak(s).
#include <vector>
int main()
{
std::vector<double> a;
int n = 5;
a.resize(n);
return 0;
}
I have the following class in C++:
class a {
const int b[2];
// other stuff follows
// and here's the constructor
a(void);
}
The question is, how do I initialize b in the initialization list, given that I can't initialize it inside the body of the function of the constructor, because b is const?
This doesn't work:
a::a(void) :
b([2,3])
{
// other initialization stuff
}
Edit: The case in point is when I can have different values for b for different instances, but the values are known to be constant for the lifetime of the instance.
With C++11 the answer to this question has now changed and you can in fact do:
struct a {
const int b[2];
// other bits follow
// and here's the constructor
a();
};
a::a() :
b{2,3}
{
// other constructor work
}
int main() {
a a;
}
Like the others said, ISO C++ doesn't support that. But you can workaround it. Just use std::vector instead.
int* a = new int[N];
// fill a
class C {
const std::vector<int> v;
public:
C():v(a, a+N) {}
};
It is not possible in the current standard. I believe you'll be able to do this in C++0x using initializer lists (see A Brief Look at C++0x, by Bjarne Stroustrup, for more information about initializer lists and other nice C++0x features).
std::vector uses the heap. Geez, what a waste that would be just for the sake of a const sanity-check. The point of std::vector is dynamic growth at run-time, not any old syntax checking that should be done at compile-time. If you're not going to grow then create a class to wrap a normal array.
#include <stdio.h>
template <class Type, size_t MaxLength>
class ConstFixedSizeArrayFiller {
private:
size_t length;
public:
ConstFixedSizeArrayFiller() : length(0) {
}
virtual ~ConstFixedSizeArrayFiller() {
}
virtual void Fill(Type *array) = 0;
protected:
void add_element(Type *array, const Type & element)
{
if(length >= MaxLength) {
// todo: throw more appropriate out-of-bounds exception
throw 0;
}
array[length] = element;
length++;
}
};
template <class Type, size_t Length>
class ConstFixedSizeArray {
private:
Type array[Length];
public:
explicit ConstFixedSizeArray(
ConstFixedSizeArrayFiller<Type, Length> & filler
) {
filler.Fill(array);
}
const Type *Array() const {
return array;
}
size_t ArrayLength() const {
return Length;
}
};
class a {
private:
class b_filler : public ConstFixedSizeArrayFiller<int, 2> {
public:
virtual ~b_filler() {
}
virtual void Fill(int *array) {
add_element(array, 87);
add_element(array, 96);
}
};
const ConstFixedSizeArray<int, 2> b;
public:
a(void) : b(b_filler()) {
}
void print_items() {
size_t i;
for(i = 0; i < b.ArrayLength(); i++)
{
printf("%d\n", b.Array()[i]);
}
}
};
int main()
{
a x;
x.print_items();
return 0;
}
ConstFixedSizeArrayFiller and ConstFixedSizeArray are reusable.
The first allows run-time bounds checking while initializing the array (same as a vector might), which can later become const after this initialization.
The second allows the array to be allocated inside another object, which could be on the heap or simply the stack if that's where the object is. There's no waste of time allocating from the heap. It also performs compile-time const checking on the array.
b_filler is a tiny private class to provide the initialization values. The size of the array is checked at compile-time with the template arguments, so there's no chance of going out of bounds.
I'm sure there are more exotic ways to modify this. This is an initial stab. I think you can pretty much make up for any of the compiler's shortcoming with classes.
ISO standard C++ doesn't let you do this. If it did, the syntax would probably be:
a::a(void) :
b({2,3})
{
// other initialization stuff
}
Or something along those lines. From your question it actually sounds like what you want is a constant class (aka static) member that is the array. C++ does let you do this. Like so:
#include <iostream>
class A
{
public:
A();
static const int a[2];
};
const int A::a[2] = {0, 1};
A::A()
{
}
int main (int argc, char * const argv[])
{
std::cout << "A::a => " << A::a[0] << ", " << A::a[1] << "\n";
return 0;
}
The output being:
A::a => 0, 1
Now of course since this is a static class member it is the same for every instance of class A. If that is not what you want, ie you want each instance of A to have different element values in the array a then you're making the mistake of trying to make the array const to begin with. You should just be doing this:
#include <iostream>
class A
{
public:
A();
int a[2];
};
A::A()
{
a[0] = 9; // or some calculation
a[1] = 10; // or some calculation
}
int main (int argc, char * const argv[])
{
A v;
std::cout << "v.a => " << v.a[0] << ", " << v.a[1] << "\n";
return 0;
}
Where I've a constant array, it's always been done as static. If you can accept that, this code should compile and run.
#include <stdio.h>
#include <stdlib.h>
class a {
static const int b[2];
public:
a(void) {
for(int i = 0; i < 2; i++) {
printf("b[%d] = [%d]\n", i, b[i]);
}
}
};
const int a::b[2] = { 4, 2 };
int main(int argc, char **argv)
{
a foo;
return 0;
}
You can't do that from the initialization list,
Have a look at this:
http://www.cprogramming.com/tutorial/initialization-lists-c++.html
:)
A solution without using the heap with std::vector is to use boost::array, though you can't initialize array members directly in the constructor.
#include <boost/array.hpp>
const boost::array<int, 2> aa={ { 2, 3} };
class A {
const boost::array<int, 2> b;
A():b(aa){};
};
How about emulating a const array via an accessor function? It's non-static (as you requested), and it doesn't require stl or any other library:
class a {
int privateB[2];
public:
a(int b0,b1) { privateB[0]=b0; privateB[1]=b1; }
int b(const int idx) { return privateB[idx]; }
}
Because a::privateB is private, it is effectively constant outside a::, and you can access it similar to an array, e.g.
a aobj(2,3); // initialize "constant array" b[]
n = aobj.b(1); // read b[1] (write impossible from here)
If you are willing to use a pair of classes, you could additionally protect privateB from member functions. This could be done by inheriting a; but I think I prefer John Harrison's comp.lang.c++ post using a const class.
interestingly, in C# you have the keyword const that translates to C++'s static const, as opposed to readonly which can be only set at constructors and initializations, even by non-constants, ex:
readonly DateTime a = DateTime.Now;
I agree, if you have a const pre-defined array you might as well make it static.
At that point you can use this interesting syntax:
//in header file
class a{
static const int SIZE;
static const char array[][10];
};
//in cpp file:
const int a::SIZE = 5;
const char array[SIZE][10] = {"hello", "cruel","world","goodbye", "!"};
however, I did not find a way around the constant '10'. The reason is clear though, it needs it to know how to perform accessing to the array. A possible alternative is to use #define, but I dislike that method and I #undef at the end of the header, with a comment to edit there at CPP as well in case if a change.